Airway pressure release ventilation (APRV) is a mode of ventilation believed to offer advantages as a lung protective ventilator strategy. APRV is a form of continuous positive airway pressure (CPAP) with an intermittent release phase from a preset CPAP level. APRV allows maintenance of substantially constant airway pressure to optimize end inspiratory pressure and lung recruitment. The CPAP level optimizes lung recruitment to prevent or limit low volume lung injury. In addition, the CPAP level provides a preset pressure limit to prevent or limit over distension and high volume lung injury. The intermittent release from the CPAP level augments alveolar ventilation. Intermittent CPAP release accomplishes ventilation by lowering airway pressure. In contrast, conventional ventilation elevates airway pressure for tidal ventilation. Elevating airway pressure for ventilation increases lung volume towards total lung capacity (TLC), approaching or exceeding the upper inflection point. Limiting ventilation below the upper inflection of the P-V (airway pressure versus volume) curve is one of the goals of lung protective strategies. Subsequently, tidal volume reduction is necessary to limit the potential for over distension. Tidal volume reduction produces alveolar hypoventilation and elevated carbon dioxide levels. Reduced alveolar ventilation from tidal volume reduction has lead to a strategy to increase respiratory frequency to avoid the adverse effects of hypercapnia. However, increased respiratory frequency is associated with increase lung injury. In addition, increase in respiratory frequency decreases inspiratory time and the potential for recruitment. Furthermore, increasing respiratory frequency increases frequency dependency and decreases potential to perform ventilation on the expiratory limb of the P-V curve.
During APRV, ventilation occurs on the expiratory limb. The resultant expiratory tidal volume decreases lung volume, eliminating the need to elevate end inspiratory pressure above the upper inflection point. Therefore, tidal volume reduction is unnecessary. CPAP levels can be set with the goal of optimizing recruitment without increasing the potential for over distension. Consequently, end inspiratory pressure can be limited despite more complete recruitment and ventilation can be maintained.
Airway pressure release ventilation (APRV) was developed to provide ventilator support to patients with respiratory failure. Clinical use of APRV is associated with decreased airway pressures, decreased dead space ventilation and lower intra-pulmonary shunting as compared to conventional volume and pressure cycled ventilation. APRV limits excessive distension of lung units, thereby decreasing the potential for ventilator induced lung injury (VILI), a form of lung stress. In addition, APRV reduces minute ventilation requirements, allows spontaneous breathing efforts and improves cardiac output. APRV is also associated with reduction or elimination of sedative, inotropic and neuromuscular blocking agents.
APRV is a form of positive pressure ventilation that augments alveolar ventilation and lowers peak airway pressure. Published data on APRV has documented airway pressure reduction on the order of 30 to 75 percent over conventional volume and pressure cycled ventilation during experimental and clinical studies. Such reduction of airway pressure may reduce the risk of VILI. APRV improves ventilation to perfusion ratio (VA/Q) matching and reduces shunt fraction compared to conventional ventilation. Studies performed utilizing multiple inert gas dilution and excretion technique (MIGET) have demonstrated less shunt fraction, and dead space ventilation. Such studies suggest that APRV is associated with more uniform distribution of inspired gas and less dead space ventilation than conventional positive pressure ventilation.
APRV has been associated with improved hemodynamics. In a 10-year review of APRV, Calzia reported no adverse hernodynamic effects. Several studies have documented improved cardiac output, blood pressure and oxygen delivery. Consideration of APRV as an alternative to pharmacological or fluid therapy in the hemodynamically-compromised, mechanically-ventilated patient has been recommended in several case reports.
APRV is a spontaneous mode of ventilation which allows unrestricted breathing effort at any time during the ventilator cycle. Spontaneous breathing in Acute Respiratory Distress Syndrome/Acute Lung Injury (ALI/ARDS) has been associated with improved ventilation and perfusion, decreased dead space ventilation and improved cardiac output and oxygen delivery. ALI/ARDS is a pathological condition characterized by marked increase in respiratory elastance and resistance. However, most patients with ALI/ARDS exhibit expiratory flow limitations. Expiratory flow limitations results in dynamic hyperinflation and intrinsic positive end expiratory pressure (PEEP) development. In addition, ARDS patients experience increased flow resistance from external ventilator valving and gas flow path circuitry including the endotracheal tube and the external application of PEEP.
Several mechanisms can induce expiratory flow limitations in ALI/ARDS. In ALI/ARDS both FRC and expiratory flow reserve is reduced. Pulmonary edema development and superimposed pressure result in increased airway closing volume and trapped volume. In addition, the reduced number of functional lung units (de-recruited lung units and enhanced airway closure) decrease expiratory flow reserve further. Low volume ventilation promotes small airway closure and gas trapping. In addition, elevated levels of PEEP increase expiratory flow resistance. In addition to downstream resistance, maximal expiratory flow depends on lung volume. The elastic recoil pressure stored in the proceeding lung inflation determines the rate of passive lung deflation.
APRV expiratory flow is enhanced by utilization of an open breathing system and use of low (0-5 cmH2O) end expiratory pressure. Ventilation on the expiratory limb of the P-V curve allows lower PEEP levels to prevent airway closure. Lower PEEP levels result when PEEP is utilized to prevent de-recruitment rather than attempting partial recruitment. Increasing PEEP levels increases expiratory resistance, conversely lower PEEP reduce expiratory resistance, thereby accelerating expiratory flow rates. Sustained inflation results in increased lung recruitment (increased functional lung units and increased recoil pressure) and ventilation along the expiratory limb (reduced PEEP and expiratory flow resistance), improving expiratory flow reserve. In addition, release from a sustained high volume increases stored energy and recoil potential, further accelerating expiratory flow rates. Unlike low volume ventilation, release from a high lung volume increases airway caliber and reduces downstream resistance. Maintenance of end expiratory lung volume (EELV) to inflection point of the flow volume curve and the use of an open system allows reduction in circuitry flow resistance. EELV is maintained by limiting the release time and titrated to the inflection point of the flow volume curve. Reduced levels of end expiratory pressure are required when ventilation occurs on the expiratory limb of the P-V curve. In ALI/ARDS, increased capillary permeability results in lung edema. Exudation from the intravascular space accumulates, and superimposed pressure on dependent lung regions increases and compresses airspaces. Dependent airspace collapse and compressive atelectasis results in severe VA/Q mismatching and shunting. Regional transpulmonary pressure gradients which exist in the normal lung are exaggerated during the edematous phase of ALI/ARDS. Patients typically being in the supine position, forces directed dorsally and cephalad progressively increase pleural pressures in dependent lung regions. Ventilation decreases as pleural pressure surrounding the dependent regions lowers transalveolar pressure differentials. Full ventilatory support during controlled ventilation promotes formation of dependent atelectasis, increase VA/Q mismatching and intrapulmonary shunting. Increasing airway pressure can re-establish dependent trans-pulmonary pressure differential but at the risk of over distension of nondependent lung units. Alternatively, spontaneous breathing, as with APRV, can increase dependent transpulmonary pressure differentials without increasing airway pressure.
APRV allows unrestricted spontaneous breathing during any phase of the mechanical ventilator cycle. As noted, spontaneous breathing can lower pleural pressure, thereby increasing dependent transpulmonary pressure gradients without additional airway pressure. Increasing dependent transpulmonary pressure gradients improves recruitment and decreases VA/Q mismatching and shunt. As compared to pressure support ventilation (PSV) multiple inert gas dilution technique, APRV provides spontaneous breathing and improved VA/Q matching, intrapulmonary shunting and dead space. In addition, APRV with spontaneous breathing increased cardiac output. However, spontaneous breathing during pressure support ventilation was not associated with improved VA/Q matching in the dependent lung units. PSV required significant increases in pressure support levels (airway pressure) to match the same minute ventilation.
Conventional lung protective strategies are associated with increased use of sedative agents and neuromuscular blocking agents (NMBA). The increased use of sedative and NMBA may increase the time the patient must remain on mechanical ventilation (“vent days”) and increase complications. NMBA are associated with prolonged paralysis and potential for nosocomial pneumonia. APRV is a form of CPAP and requires spontaneous breathing.
Decreased usage of sedation and neuromuscular blocking agents (NMBA) has been reported with APRV. In some institutions, APRV has nearly eliminated the use of NMBA, resulting in a significant reduction in drug costs. In addition to drug cost reduction, elimination of NMBA is thought to reduce the likelihood of associated complications such as prolonged paralysis and may facilitate weaning from mechanical ventilation.
Mechanical ventilation remains the mainstay management for acute respiratory failure. However, recent studies suggest that mechanical ventilation may produce, sustain or increase the risk of acute lung injury (ALI). Ventilator induced lung injury (VILI) is a form of lung stress failure associated with mechanical ventilation and acute lung injury. Animal data suggest that lung stress failure from VILI may result from high or low volume ventilation. High volume stress failure is a type of stretch injury, resulting from over distension of airspaces. In contrast, shear force stress from repetitive airway closure during the tidal cycle from mechanical ventilation results in low volume lung injury.
Initially, lung protective strategy focused on low tidal volume ventilation to limit excessive distension and VILI. Amato in 1995 and in 1998 utilized lung protective strategy based on the pressure-volume (P-V) curve of the respiratory system. Low tidal volumes (6 ml/kg) confined ventilation between the upper and lower inflection points of the P-V curve. End expiratory lung volume was maintained by setting PEEP levels to 2 cmH2O above the lower inflection point. Amato demonstrated improved survival and increased ventilator free days.
However, subsequent studies by Stewart and Bower were unable to demonstrate improved survival or ventilator free days utilizing low tidal volume ventilation strategy. Unlike Stewart and Bower, Amato utilized elevated end expiratory pressure in addition to tidal volume reduction. Such important differences between these studies limited conclusions as to the effectiveness of low tidal ventilation limiting ventilator associated lung injury (VALI).
Recent completion of the large controlled randomized ARDSNet trial documented improved survival and ventilator free days utilizing low tidal volume ventilation (6 ml/kg) vs. traditional tidal volume ventilation (12 ml/kg). Although the low tidal volume group (6 ml/kg) and traditional tidal volume group (12 ml/kg) groups utilized identical PEEP/FiO2 scales, PEEP levels were significantly higher in the low tidal volume group. Higher PEEP levels were required in the low tidal volume group in order to meet oxygenation goals of the study. Despite improved survival in the low tidal volume group (6 ml/kg) over traditional tidal volume group (12 ml/kg), survival was higher in the Amato study. The ARDSNet trial also failed to demonstrate any difference in the incidence of barotrauma. The higher PEEP requirements and the potential for significant intrinsic PEEP from higher respiratory frequency in the lower tidal volume group, may have obscured potential contribution of elevated end expiratory pressure on survival. Further studies are contemplated to address the issue of elevated end expiratory pressure.
In the prior art, utilization of the quasi-static inspiratory pressure versus volume (P-V) curve has been advocated as the basis for controlling a ventilator to carry out mechanical ventilation. The shape of the inspiratory P-V curve is sigmoidal and is described as having three segments. The curve forms an upward concavity at low inflation pressure and a downward concavity at higher inflation pressures. Between the lower concavity and the upper concavity is the “linear” portion of the curve. The pressure point resulting in rapid transition to the linear portion of the curve has been termed the “lower inflection point”. The lower inflection point is thought to represent recruitment of atelectatic alveolar units. The increasing slope of the P-V curve above lower inflection point reflects alveolar compliance. Above the inflection point, the majority of air spaces are opened or “recruited”. Utilizing the lower inflection point of the inspiratory P-V curve plus 2 cmH2O has been proposed to optimize alveolar recruitment. Optimizing lung recruitment prevents tidal recruitment/de-recruitment and cyclic airway closure at end expiration. Ultimately, optimizing lung recruitment could potentially reduce shear force generation and low volume lung injury.